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Road Design & Traffic Engineering

3.6 Traffic Calming

Traffic calming measures can be very effective in reducing the number of injury collisions, especially in residential areas. However, motorcyclists are no more exempt from the intended effects of traffic calming than any other road user and, arguably, suffer disproportionately from the unintended effects. These unintended effects stem from issues that can be grouped under three headings: design, materials and maintenance. The key point to remember is that riding a motorcycle is a permanent balancing act and that the vehicle has only two small tyre “footprints” to provide grip. One of the first issues facing the designer of a traffic calming scheme should be whether to use physical traffic calming measures or not. The designer should examine the collision history for the area in order to define the objectives of a traffic calming scheme and to determine what level of speed reduction is appropriate in order to address the problems identified. Alternatively, more subtle changes to the environment, using existing environmental features and taking a holistic approach to the street scene, may be appropriate. It may be possible to adopt a psychological approach to traffic calming methods by modifying the built environment in such a way as to create a ‘negotiated space’ for all road users, with a concomitant reduction in speed and thereby enable the reduction or even removal of the need for vertical traffic calming features. The choice of what type of physical measures to use is normally influenced by a number of factors, including:

  • collision records
  • traffic volume
  • pedestrian activity
  • target speed
  • the presence of bus routes
  • the needs of emergency services
  • sensitivity of the local environment
  • the views of local residents
  • the budget.

Many motorcyclists support the use of speed cushions which give the option of riding between the speed cushions rather than over them. This does not negate the measure; the act of aligning a motorcycle to pass through a small gap between speed cushions will bring about a natural reduction in speed, although it may fall short of the target speed for the scheme. Useful publications in this area include Home Zone Design Guidelines (IHIE 2002) and Traffic Calming Techniques (IHT/CSS 2005) and the Traffic Advisory Leaflet 01/07 “Traffic Calming”.  LTN1/07 provides a good summary of the different types of measures and a section on traffic calming and motorcyclists. It also offers a comprehensive summary of the research commissioned by the Department for Transport and external sources, to be used as guidance on traffic calming measures today.  It covers relevant legislation, design, effectiveness and installation.

3.6.1 Location of Traffic Calming Measures

A major problem facing the designer of traffic calming schemes is choosing the location of the measures. The needs and vulnerabilities of motorcyclists should be accounted for along with all the other factors influencing this process. Some important issues are:

  • Vertical traffic calming measures should not be located anywhere a motorcyclist will need to brake or change direction (i.e. not leaning over). Ramps for raised junctions should begin far enough back so that the motorcycle negotiates the ramp in a straight line.
  • The design of the scheme should include adequate warning signs, both permanent and temporary, warning of the new road layout ahead. Consider using on-site publicity prior to installing the traffic calming measures but any temporary signs should be removed no later than three months after the completion of the scheme.
  • Whenever possible a non-vertical speed reducing feature should be used at the entrance to a traffic calming scheme. Following changes in the regulations affecting traffic calming scheme design, some schemes have emerged that start with a speed hump or cushions. Without some form of non-vertical speed-reducing feature, there is always the risk that a motorcyclist will unintentionally hit the first vertical measure at speed. This could result in an uncomfortable bump for car driver, but a serious injury for a motorcyclist.
  • A commonly used non-vertical speed reducing feature in traffic calming schemes is the mini-roundabout. Without careful design, motorcyclists, like other vulnerable road users, can suffer disproportionate risk at these junctions. Particular points to bear in mind include the following:
    • Ensure adequate skidding resistance on the mini-roundabout central area and arrow markings.
    • Most junction designs are checked to make sure that larger vehicles’ swept paths can be accommodated. Motorcycles making tight right turns at mini-roundabouts can have stability problems, especially if the turn is more than 90°.
    • Make sure there is adequate advanced warning of the junction type.
    • Ensure adequate visibility; using a mini-roundabout because there is insufficient visibility for a priority junction is rarely a safe option for any road user.
  • Mini roundabouts should not be used on roads with speed limits of more than 30mph and 4-arm mini roundabouts should be avoided due to the over representation of bicycle and motorcycle collisions.  DMRB TD 54/07 sets out specific design considerations for mini roundabouts and, in particular, makes relevant recommendations for motorcycles.
  • The design of horizontal schemes, often in the form of chicanes, should allow for the fact that motorcyclists on lower-powered machines tend to ride near the kerb. The build-outs often associated with these schemes can pose the following risks:
    • They can catch riders unaware, leading to collision. This is especially a problem for new schemes. Build-outs should be conspicuous.
    • They may force motorcyclists to move to the centre of the road or even, in priority working schemes, into oncoming traffic, much of which is unwilling to give way to a motorcycle.
    • The use of low over-run areas as build-outs can cause stability problems if a rider clips the edge. The standards permit an up-stand of no more than 15mm but even this can cause problems to the rider of a smaller machine when turning.
  • Islands or refuges used to reduce the width of the road should be conspicuous, allowing motorcyclists to position themselves correctly in advance, avoiding late and sudden changes of direction.
  • Always ensure the location of traffic calming measures does not lead to poor drainage; standing surface water could compromise motorcycle safety, especially in freezing conditions.
  • Humps and speed cushions should be located 10-15 metres away from junctions to allow riders to turn out and to pass over them without leaning over. This should be balanced against pedestrian desire lines where flat-topped humps are being specifically installed as crossings.
  • Bicycle bypasses around traffic calming measures should be clearly marked in order to discourage use by motorcycles.
  • Block paving treatments often have uneven characteristics and do not have the same friction value as the surrounding road surface.  This is very important to remember at or near critical junctions.

3.6.2 Materials

There is an array of choices facing the designer when selecting materials for traffic calming measures.  Influential factors may relate to the local environment, technical performance, maintenance, finances and purchasing policy.  The safety of road users, including motorcyclists, must be equally influential. Particular issues to consider include the following:

  • The use of block paving or stone setts, including on speed tables and raised junctions. These often have poor skid resistance, especially when wet. They are hard to maintain and displaced blocks and alternative reinstatement materials result in uneven and unpredictable surfaces for motorcyclists.
  • Pre-cast concrete traffic calming features can crack or rock and become displaced if not properly laid. Pre-formed rubber features can also deform over time and lift at the edges.  This presents a hazard for motorcyclists and, during the winter, is difficult to treat for ice prevention.
  • The use of white thermoplastic on traffic calming measures.  White triangles used to highlight humps, for example, are mandatory but can cause problems when:
    • The material used has insufficient skid resistance. Markings are often at the centre of a lane, just where the wheel tracks of motorcycles will pass.
    • High traffic levels or poorly specified material can cause them to fade quickly, making a bitumen-based measure hard to distinguish against the road surface, especially at night.
  • Transverse bars, rumble strips and “dragon’s teeth” markings on the approach to gateways and other traffic calming measures are often in the braking zone for road users. If the materials do not provide suitable skid resistance or if they present a series of vertical displacements they can constitute an added hazard for motorcyclists.
  • The potential to use collapsible or frangible street furniture, especially in locations that could conceivably be in the path of a falling rider. 
  • The use of bitumen to seal cracks and reinstatements. This material has very low skid resistance in wet conditions, yet is routinely used to seal the joints in traffic calming measures, particularly speed cushions. Alternative materials should be specified where possible.

3.6.3 Lighting

Street lighting is a specialised subject requiring an increasingly sophisticated approach and comprehensive knowledge of the types of light sources available and the best places to use them. It is of vital importance that the night-time safety and performance of traffic calming measures are not compromised by inadequate street lighting. Motorcycle headlamps are typically less bright than those of other vehicles and therefore good lighting within the scheme is crucial. Obviously the first point of contact will be the local authority street lighting team; if this is not possible, then the Institution of Lighting Engineers (ILE) can provide advice on the technical aspects of scheme lighting to suit all road users, including motorcyclists. The ILE has published a technical report called Lighting for Traffic Calming Features (ILE 2002). Bear in mind the following particular issues:

  • The scheme should be evenly illuminated, with particular attention being paid to traffic calming measures that alter the normal road alignment, for example build-outs and chicanes.
  • The lighting should provide good colour rendering, especially where colour is an integral part of the scheme.

3.6.4 Maintenance

The first traffic calming schemes were installed on public roads in the UK in the 1980s and maintenance of these older schemes is now becoming a problem for many local authorities across the country. Poorly maintained traffic calming schemes can be a hazard to both road users and pedestrians but the consequences will often be most severe for motorcyclists.  Maintenance issues in general are dealt with in Chapter 7, but, in relation to traffic calming schemes, be alert to:

  • Reinstatement using non-original materials that presents an inconsistent road surface to riders.
  • Uneven wear on vertical measures (especially where block paving, pre-cast concrete or rubber is used) leading to unexpected depressions.
  • Road markings that fall below acceptable standards for retro-reflectivity and skid resistance (assuming they met them when new) or even fade away completely.